Greene



L. GREENE STALL WARNING DEVICE FOR AIRPLANES Dec. 5, 1950 2 Sheets-Sheet1 Original Filed May 12, 1944 INVENTOR. LEON/4RD GREENE ATTOKNEK ll.GREENE STALL WARNING DEVICE FOR AIRPLANES Dec. 5, 1950 Original FiledMay 12', 1944 2 Sheets-Sheet 2 INVENTOR. LEON/QED GREENE.

Reissued Dec. 5, 1950 UNITED OFFICE llietter'enclosed-iiniheavytbracketsl ihappcars im'the original patentbut: formsno. partof this reissuespecificatiom: matter .printedriim'itafiesindicatesi'theraidditienswade .by. reissue This invention relates .to.airplanes-landcmoise particularly to means; for indicating .theanglewofattackcdm'ings flight; forithegpurposecof .--warning of.-theapproach::ofastall.

The forces to which;aaneairlplanelisesubjected consist-of gravity,ecceleration,.ithrust sdnagmmd lift. The ability of -..the'.wings.t0umaintaintthe airplane in flight other than-inn stalledcondition,ismeasured :by. the lift whichtthatxwing is. capable of' producing.The-liftiofsa=wingemusteequalsthe forcesexerted-iorr it attall. times.:liftcanLhe expressed ima:iormulacwherein-the.liftis:equaleto the wing.areamultiplieduby.the:eoeificient:oti'liit, asconstant; theidensity of.-the.;air;and...thesquare otthe velocity.

In the-formula...L=Ac1= (kD) '=V?,L:-is the1-1ift, A thelarea ofthexwingzsnrfiacepm isea coeflicient of..'lift,'-- lcis a'enumericalconstanth'llisthedensity of: the: air, V is. the :vecity=; ofthe-airplane.

this-equation, the..-areaz.-of..the wingiiskncwn .-or may readilybeadeterminecl. Theidensity :of .the

air: in which: the. plane :iS"..'.'flOWI1,-the velocity-get whichvit isifiown, andt the :vertical;'iorcesiorfiliit which are.imposedazupon..theexvingpall.armcondi- .tions whichcthegpilot. ofthe-airplanesrletermines during flight. It isiitheicoetficientlofliftlinzthe above formula which-varies-so'as. to maintain therelationship.:expressed:.:by'sthe: formula. iIf this coeifici'ent f;lift were.:.capable. of evening :lwithout limit; .then.any:conditions:=of flight would be I metzbysa eorrespondi-ng'variatiortinziahe valneeof 01. This. is not.sogrhoweverg-as. the coeilicientofliftmasia maximum value-.whicliis predetermined by thezdesignot-thewing.

*Themoefficient 'of liitawillvarysolelyuwith the angle oi-attackof .thewing. .-..-In. :defining anglesof attack,= zwe may say; thatJthis:-anglei.-==is. the. angle formed-by. the chord' of. thawing-1am!the. direction of the air. through whichl it moves. Thisxvarietion ofthe coeiiicient of li-ft with'ltheanglewonattackcan 'be predetermined bymeans-oof'ztests. For any-wing section-there. isan angle o'f-:attaekwhich oreates the maximums coefilcient :of lifi. Ifl'th-is angle ofattach-"is" exceededl-theicoefllcient of lift will decrease; and. theairplane will stall. This angle of attack which creates a: maximumcoefficient of lift is called the stalling angleolattack and its valueis 'knownior can be determined for any airplane.

The present invention contemplates the provision of a. device forindicating-an: angle onsttack equal to the-above stated critical value.:The invention also contemplates lndication of an angle of-attackslightly less than said critical value-to give warning inedvance of 1 animpending stall so 42 .thatz rectiiicationof.the'condition which iscreatingtthestailin 3316. of. attack, may be made.

Z'lheiedvantages otian indicating device for this ;puLpose-:-shouldbesapparent-lwhen-it is pointed :ouhthataatethe stalling angle-. 01"attack, the co- 5 ?fl i tiof fizmling.is .the' maximum for that.wingz-surface. Hence,.:any. further decrease in velocity..or;increasee;ini load due to acceleration, -WiuiiBXEEt a. force; on:.the- -.Wing ;greater than that which the wing can maintain and-yetkeepthe airplane. fsemzbecoming. uncontrollable. In givingthin-warning.zbyzrindicating thezapproach of the stalling angle ofattack, the: possibility of obtainingra point .where. a:coefiicient"oflift is, demanded ontheaswingtsuriace in excess of .that which it cansupplynby. virtue:.of..-its :design, is eliminated.

With v the; foregoinguin mind, the objects, features; amid -advantages;of: the. invention will become: moreuclear-lyeapparent from the.following 'detailed specification which has basis on theaccompanyingidrawing in which the invention is examplified.

ElFig; 1 is a diagrammatic view showing. the flow lines. abont annirfoilwhich is. set .at a small angle with respect tothe flow direction.

FigiZ islalsimilar view but showing the airfoil .ataa 'greateranglewithrespect tothe flow disection.

*Ffigsifland iaare diagrammatic views of an -edge=of' an -aa-irplanewingshowing the two positions on the device of the invention. withrespect to the ilow dire'ction.

Figi-i'wis aside view ofan embodiment of the invention-with its vane inposition it assumes in normal alight i -lest isa; plan view thereof.

Fige 'l' is a view' similar to Fig. 5 but showing the vane'in theposition it assumes when warning of-an-approaching stall.

Fig.'8-is an'edge view of anairplane-wingincorporatinganother embodimentof the invention and shownin normal flight.

Fig. 94s a similarview thereof in which the angle of attack is critical.

When-an-airfoil-is moved throughan air mass titan-angle which'al'lowsthe air to flow smoothly over its surfaces; the air-is separated by theairfell into two portions which follow the upper and lowersurfaees'respectively. This invention takes place in the vicinity of theleading edge of the aicfoil. *This point -Where the flow divides isknown as the stagnation point and the air -immedlatclylabove andibelowittravels in opposite directions away from this point.

In Fig. l is illustrated diagrammatically the flow lines about anairfoil which is set at a small angle with respect to the flowdirection, while in Fig. 2 the flow lines are shown in the position theyassume when the angle of attack, or the angle with respect to the flowdirection, has been increased. The point marked S is the stagnationpoint (point where the flow divides) and it is seen that increasing theangle of the airfoil moves this point down and aft along the lowersurface of the airfoil.

It is a characteristic of airfoils that the lift increases as the angleof attack is increased but only up to a certain angle of attack. If theairfoil assumes an angle greater than this value, the flow ceases tofollow the curved upper surface and what is known as a stall takesplace. This stall is accompanied by a loss of lift and constitutes anunsafe flight condition unless the pilot is aware of the condition.

The change in stagnation point position which accompanies change ofangle of attack, may be utilized as a means whereby the pilot may bewarned of an approaching stall condition. If a small swinging vane orflap be attached to, or adjacent to, the zone of the stagnationpoint,.the changing direction of flow and wind force on the vane, whenthe airfoil is at an angle of attack near the stall condition, willprovide a stallwarning means.

The operation of such a stall-warning mechanism is described as follows:when the angle of attack is low and corresponds to a safe flightcondition, the stagnation point is above the vane, as indicated in Fig.3. In this condition, the air is moving down from the stagnation pointand causes a downward force on the vane. This will produce no warningsignal. However, if the angle of attack is increased beyond that whichis considered safe, the stagnation point will be located below and aftof the vane, and this will cause an upward flow and force the vaneupwardly so that it may close an electric circuit and be relayed bysignal means to, the pilot as a warning of an approaching stallcondition.

Measurement of the position of the stagnation point with respect to theleading edge of an airfoil offers adistinct advantage as a stall-warningmeans. As the stagnation point moves from a position above the vane to aposition below it,

the complete change in flow direction offers a sensitive and positivestall-warning means. Potential flow theory, whereby flow about anairfoil is treated mathematically through conformal transformation,shows that the location ofthe stagnation point has a definite relationto 'the lift of an airfoil. Flight tests have proven that this factprovides a reliable stall-warning means.

The illustrated form of the invention is applied to the leading edge ofa wing 5 and comprises a vane or flap 6 pivoted to said wingas at I. Thevane or flap B is supported by a'bracket 8 so that in normal flight(Fig. 8) an angle a is formed between said flap and an extension of thechord great with respect to the direction of the air mass, the flap 6,still resting on the bracket 8, is angularly directed so that the airmass, indicated by the arrows Ifla of Fig. 9, impinges on the undersurface of the flap. The air mass will then lift the flap on its pivot 1to decrease the angle a to the angle a and be brought into engagementwith the contact H to light the Warning lamp 1? arranged in an electriccircuit including a power source 13, the flap 6, and the contact H. Thiswarning light or other visual or audible signaling means may be locatedin the cock-pit or cabin for observation by the airplane pilot. Uponreceiving the warning that the stalling angle of attack is beingapproached, the pilot may take such action as is necessary to bring theairplane to a safer flying condition. As soon .as the angle of the chord9, with the air mass,

is decreased, the flap will again fall upon the bracket 8 to wipe outthe signal and thus indicate that the airplane is being safely directed.

The angle a may be arranged in accordance with the degree of safetydesired and in accordance with the design and characteristics of thewing to which it is applied.

The angle of attack, as previously defined, is solely the angle made bythe wing or air mass vector with the chord of the wing and, theree fore,.is independent of the attitude or angular disposition of the airplanewith respect to the earth. For example, if the chord of the Wingwere-inclined so as to be vertical and at an angle of 96 with respect tothe earth, and at the same time, the air passing the wing were in linewith the chord, the attitude of the plane would be with respect to theearth, but would be 0 with respect to the air through which it ismoving. No signal would be given under these conditions because the windwould be striking the vane on its upper surface and, therefore, wouldkeep it pressed against its supporting bracket, Not giving a warningsignal under this condition is de-. sired, as this condition occurs inseveral acrobatic maneuvers, such as during a loop, Immelman, wingover,etc., and the airplane is not in a stalled condition. On the other hand,if the airplane were pointed downward with respect to the horison, andat the same time the forces exerted onthe wing cause the. wing to moveto an angle of attack with resepct to the air exceeding the angle atwhich the vane was supported, the wind would strike. the under surfaceof the vane and lift it. to close the contact to the electric warn-. ingcircuit. This would show a condition exceeding the safety margin despitethe fact that the attitude of the airplane is downward. It is desirousto have the device give a warning sig-s nal under this condition as thiscondition occurs during a glide where the pilot attempts to glide toofar without a suflicient decrease in alti-' tude, or it occurs when thepilot attempts to raise the nose of the airplane at too low a speed. Inthese situations the flight of the airplane is close to. a dangerousstall or tailspin. These extremes are given to show the independentnature of the angle'of attack as compared to the attitude of theairplane.

To demonstrate the critical or unstable equilibrium which exists at thestalling angle of attack, it can be stated that when this angle ofattack is exceeded, the coeflicient of lift, instead of increasing, willdecrease. Therefore, from our original formula, this lift will not beable to cause an equilibrium with the other forces on the airplane, andthe airplane will accelerate down,

wards. This 'cohclition'is lfilowii' as' a stall, and the loss ofaltitude created during a stall constitutes 'a menace to the safety offlight. This stalled ofi'ditidn' must occur "before the airp ane canbecapable "a: entering into 'a tailspin. The loss of altitude during 'a'stallor a tailspin is dangerous "because it inay bring the plane downwithin'fti' 'i'iig'dis''iice ofthe'arth. Alse the sp'd "atta ried duringthe "stall or tailspin and the direction of "flight "of the airplanewith respect to the air, create forces not contemplated in thestructural design of airplanes and may cause important structuralmembers of the airpla'i'ieto fail. Furtheh 'diiring'stalled and-spinningconditions of an airplane, the pilot has very little control over theflight of the airplane. fie eannotdnci'ease the lift 'of 'thewings' byincreasing'its angle of attack as this increase further decreasesthelift coefficient andeka'ggerates the stalledcohdition'. "This meansthataslong as a stalledortailspih condition exists,'-'the pilot hasvirtually lost all control over the motion of the plane.

Prior attempts to prevent stalled or spinning conditions entailed theuse of the air speed indicator of the airplane as a warning device.Airplanes are manufactured and designed so as to have a certain stallingspeed which is assumed to be constant. This measurement, however, is notreliable as this stalling speed is not constant under all conditions. Infact, it varies in almost unlimited degree. For example, if a givenairplane has a stalling speed of miles per hour in straight and levelunaccelerated flight at normal load conditions, the stalling speed of 40miles an hour will not be maintained when any of these conditions arealtered. If the airplane were to be loaded less than its maximumdesigned load for which this stalling speed was measured, the

plane will stall at a lower speed. If the forces due to acceleration,such as are created in a steep turn, are brought into account, thestalling speed may be greatly increased. An airplane in a 90 verticalturn that is not losing altitude, will have a stalling speed ofinfinity. The following table gives the variation of the stalling speedunder the above conditions at various degrees of bank in a turn:

Miles per hour 47.5 56.5 68.4 96.0 136.0 Infinity as related to thestalling point an unreliable method of indication. By means of thepresent device, and under any of the above conditions, the airplane willstall at an angle of attack which can be predetermined for anyparticular plane.

The present device may also be used to obtain two other valuableindications. First, where the bracket arm is set in line with the chordof the wingso' as'toindicate the position in which the wing is beingused in its most efiicient manner. Second, for any airplane, the angleof attack can be p're determined before flight, thecondition"depending-upon design andhorsepowerof the' airplane, so that'the maximum and most efii ci'ent' rate ofclimb may he obtained.

Under "conditions of emergency when it *is desiredto have the airplaneglidethe maximum forward distance "with 'minimum loss of altitudesuch-as in a forced landing due to motor failure, the prese'nt devicecan be used to obtain the angle of 'attack best" suited for this"condition.

---In Figs. 8 'and'9, the device has been'shownas pivoted directly *to ithe leading edge of the wing For convenience, a compact device maybe-comp'l'etely assembled as illustrated in Figs. 6*and "7 and ready tobe mortised inside of the leading edgeef a wing "withthe vane protrudingsufficiently to be affected by the air mass as the local flow direction=variesin flight.

To thisend there is shown a frame comprised of two plates orside;members 20 held in; spaced apart condition by the spacer tubes 2|extending between the plates 20 at the four corners. Pins 22 extendthrough the spacers 2| and holes in the plates 20 and the ends thereofmay be upset as at 23 to secure the frame firmly together. Instead ofpins 22 riveted in position, bolts and nuts may be used.

A freely swinging vane B is pivoted between the plates 20 as at 24 andis provided with a counterweight 25 which is so shaped that when thevane is in neutral position as in Fig. 5, the counterweight will contacta stop 26 provided within the frame. When the flap 6' is raised thecounterweight 25 will be brought into engagement with the contact I lsecured to a frame plate 28 and closes the electric circuit to light thewarning lamp or sound the audible signal.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is:

l. The combination with an airfoil having a front separation point whichshifts over the outer surface thereof with respect to a limit as theangle of attack varies, the approach to said limit substantiallycoinciding with the approach to stalling conditions, sensing meanscomprising a movable vane having at least one portion located within therange of influence of said shifting separation point and arranged tofunction on approach of said point to said stalling conditions, andindicating means controlled by said sensing means.

2. The combination with an aeronautical airfoil having a separationpoint which shifts between limits as the angle of attack varies, theapproach to one of which limits coincides with approach to stallingconditions; a vane located within the range of influence of the shiftingseparation point and arranged to function on approach of said point tothe stalling limit, and indicating means controlled by said vane.

3. A unit adapted to be mounted on the leading edge of an airfoil forwarning of an impending stall, said unit comprising a mounting meansconforming to the contour of the leading edge of the airfoil and adaptedto be mounted on said edge within the limits of fluctuation of theseparation point, a vane, means to secure said vane to said mountingmeans for pivotal movement relative thereto within the range ofinfluence of the Shifting separation point so that said vane is adaptedto be operated on'the approach of said point to stalling, conditions,said pivotal securing means being behind the center of the vane wherebythe vane is adapted to extend forwardly from the leading edge of theairfoil, a pair of spaced members between which the vane ea:- tends inorder to limit to an arc rotation of said vane in either directionrelative to said pivotal securing means, an electric switch carried bysaid mounting means and adapted to be connected to a source ofenergizing voltage, and electrically energizable indicating means havingelectric terminals adapted to be connected to said switch and to saidsource of energizing voltage whereby said vane will be operated and saidindicating means energized from said source of voltage upon movement ofthe separation point as the angle of attack approaches stallingconditions of the airfoil.

. LEONARD GREENE.

REFERENCES CITED The following references are of record in the file ofthis patent or the original patent.

8 UNITED STATES PATENTS Number Name Date 1,760,740 Branson May 27, 19302,110,730 Holland Mar. 8, 1939 2,193,077 Saxman Mar. 12, 1940 2,287 ,497Rockfeller June 23, 1942 2,328,384 Lacoe Aug. 31, 1943 2,337,753 LacoeDec. 28, 1943 2,373,089 Allen Apr. 10, 1945 FOREIGN PATENTS NumberCountry Date 346,496 Great Britain Apr. 16, 1931 OTHER REFERENCESTechnical Notes No. 670, N. A. C. A., Stall Warning Indicator,Washington, Oct. 1938.

Report No. 563, of N. A. C. A., Calculated and Measured PressureDistributions Over the Mid span Section of the N. A. C. A. 4412Airfoi1.

